We have demonstrated that corn oil gavage and feed restriction inhibit the progression of mononuclear cell leukemia and in male but not female rats. We have developed a diffusion chamber culture technique which allows us to evaluate the effect of host factors on the proliferation and differentiation of mononuclear cells. Results from these experiments reveal that circulating factors can influence the in situ proliferation of mononuclear leukemia cells. Serum isolated from animals on anti-leukemic regimens (feed restricted, corn oil, gavage, etc.) is less able to promote leukemia cell proliferation in vitro than serum from normal animals. We have identified the growth hormone: insulin like growth factor I axis as being an important mediator of the feed restriction induced inhibition of mononuclear cell leukemia.